Fracturing, or hydraulic fracturing, involves the injection of specialized fluids into geologic formations to create fractures. These fractures, in combination with flow paths drilled by wellbores, allow hydrocarbons, such as oil or natural gas, to flow from the deep recesses of the geologic formations to the ground surface. The injected specialized fluids are called fracturing fluids, which carry proppant into the fractures.
For many years, fracturing fluid technology centered on a wide range of polymers such as guar, hydroxypropyl guar (HPG) and hydroxyethylcellulose (HEC). Cross-linked polymer gels, such as those based on guar crosslinked with borate or polymers crosslinked with metal ions, were also used to attain a sufficient fluid viscosity and thermal stability in high temperature reservoirs. Since high viscosities are needed to carry the required amount of proppant, high concentrations of polymer are used. At higher polymer concentrations, several problems can occur, the most serious being unwanted residual damage to the formation face and proppant pack, plugging off possible permeable pore throats due to unbroken gels. Even with the use of advanced breaker systems, large amounts of residuals can be left behind.
Recently polymer-free viscoelastic surfactants (VES) were used for aqueous drilling and treating fluids. These VES materials are advantageous over the use of polymer gelling agents, since they are low molecular weight surfactants, in that they are less damaging to the formation, leave no filter cake on the formation face, leave very little coating on the proppant, and don't create microgels or “fish-eyes”. VES-gelled fluids are an improvement over polymer-gelled fluids from the perspective of being easier to clean up the residual gel materials after the fluid viscosity is broken and the fluid flowed back.
Viscoelastic surfactant molecules, when present at a sufficient concentration, can aggregate into overlapping worm- or rod-like micelles, which confer the necessary viscosity to the fluid to carry the proppant during fracturing. At very high shear rates, e.g., above 170 s−1, the viscosity can decrease, allowing the fluid to be pumped down the wellbore. Viscoelastic surfactant fluids are disclosed, notably, in the patents published under the numbers U.S. Pat. No. 4,615,825, U.S. Pat. No. 4,725,372, U.S. Pat. No. 4,735,731, CA-1298697, U.S. Pat. No. 5,551,516, U.S. Pat. No. 5,964,295, U.S. Pat. No. 5,979,555 and U.S. Pat. No. 6,232,274, which are all fully incorporated by reference. One well-known polymer-free aqueous fracturing fluid comprising a viscoelastic surfactant, which has been commercialized by the company group Schlumberger under the trade designation CLEARFRAC, is a mixture of the quaternary ammonium salt N-erucyl-N,N-bis(2-hydroxyethyl)-N-methyl ammonium chloride with isopropanol in a brine comprised of, for example, 3 weight percent ammonium chloride and 4 weight percent potassium chloride.
On one hand, the leak-off rate of VES fracturing fluids is normally high, so they are mainly used with hydrocarbon bearing formations wherein the permeability of the formation rocks is low. In addition, On the other hand the costs incurred by the use of high-concentration VES systems in aqueous wellbore service fluids, especially fracturing fluids, are pretty high. In particular, many VES fluid systems exhibit long viscosity recovery times after experiencing prolonged high shear. Slow recovery negatively impacts drag reduction and proppant transport capability, which consequently lead to undesirably high treating pressures and risks of near wellbore screen-outs. Many additives are developed to improve the performance but with increased cost.
Polymers and surfactants are often used together in industrial formulations to take advantage of their characteristically different properties. One class of polymers which interact particularly strongly with surfactants is the class of hydrophobically modified water-soluble polymers. See U.S. Pat. No. 4,432,881 to Evani, which is fully incorporated by reference. Since contact between the hydrophobic groups and water is unfavorable, these polymers have a strong tendency to self-associate and/or to associate with surfactants. Progressive addition of surfactants which form spherical micelles typically gives rise to an increase in the viscosity of the solution, followed by a decrease in the viscosity at higher surfactant concentrations. The enhancement in viscosity is typically attributed to the formation of mixed micelles between the polymer alkyl chains and the surfactant molecules, reinforcing polymer intermolecular cross-links. The micelles solubilize alkyl groups belonging to more than one polymer chain, and the system becomes cross-linked.
High concentrations of polymer are typically used to adequately carry an appropriate amount of proppant. At higher polymer concentrations, the fracturing fluid can have a gel-like characteristic, which can result in proppant packing and the plugging of permeable pore throats. These adverse consequences have resulted in the development and use of polymer-free viscoelastic surfactants (VESs), which have been mixed with hydrophobically modified polymers to form systems used for aqueous drilling and fracturing applications. The interaction between hydrophobically modified polymers and surfactant micelles forms a hydrocarbon-responsive fluid that breaks down upon contact with hydrocarbons. The VESs often require expensive additives to overcome VES systems that exhibit long viscosity recovery times, shearing issues, and undesirable and expensive high pressure treatment requirements.
Unfortunately, the use of fluids with hydrophobically-modified polymers in the prior art presents a significant practical limitation. Hydrophobically-modified polymers can have a slow hydration rate, making the onsite preparation of fluids containing them difficult and slow to prepare, especially at the injection well bore.
Accordingly, there is a need for fracturing fluids that are readily hydratable, have viscosity properties that enable efficient and inexpensive pumping and proppant transport down boreholes, and that reduce or avoid damage to subterranean formations harboring reservoir hydrocarbon fluids.